1. Field of the Invention
The present invention relates generally to a control device, a driving device and image forming apparatuses.
2. Description of the Related Art
Stepping motors which allow position control, speed control, and position-hold (hereinafter, “hold”) through pulse control have conventionally been used as driving sources in a large number of parts of image forming apparatuses such as copiers, facsimiles, and printers.
Stepping motors are advantageous in that position, speed, and hold are controllable through pulse control. However, to avoid loss of synchronism due to load fluctuation or change over time, a torque required of a stepping motor is larger than necessary, which disadvantageously makes stepping motors energy inefficient. Furthermore, a stepping motor capable of delivering a higher torque than required by an actual load is disadvantageously large and heavy.
In contrast, brushless direct current (DC) motors are advantageously highly efficient. This is because an electric current flowing through a brushless DC motor depends on a load. However, brushless DC motors are disadvantageous in that, unlike stepping motors, position/hold control cannot be performed.
A known example of a technique to solve these disadvantages by controlling rotational position of a brushless DC motor by adding a rotary encoder onto an output shaft of the brushless DC motor is disclosed in Japanese Laid-open Patent Publication No. 09-047056. For another example, a technique of controlling a rotational position of a brushless DC motor by adding a linear encoder onto an object to be driven by the brushless DC motor is disclosed in Japanese Laid-open Patent Publication No. 2007-097365.
However, providing a detecting device such as an encoder or a resolver on an output shaft of or an object to be driven by a brushless DC motor can cause a problem(s). Examples of the problem include occurrence of malfunction due to dust, dirt, or the like entrapped in a driving unit of the detecting device, heat-affected malfunction of the detecting device, and an increase in production cost resulting from an increase in the number of components.
Therefore, there is a need to provide a driving device and an image forming apparatus which allows position/hold control even if a detecting device is not provided on an output shaft of a brushless DC motor or an object to be driven by the brushless DC motor.